1. Field of the Invention
This invention relates generally to an antenna feed horn, and more particularly, to a compact, low weight, relatively easy to manufacture, and cost effective antenna feed horn for a satellite communications antenna array, that includes multiple chokes to provide radiation patterns with substantially equal E- and H-plane beamwidths, suppressed sidelobes, low cross-polarization, and low axial ratio across a relatively wide bandwidth or over multiple widely-separated frequency bands. Additional important features of the horn are the wide-frequency impedance match and the relatively fixed phase center from the horn aperture over a wide bandwidth.
2. Discussion of the Related Art
Various communication networks, such as Ka-band satellite communications networks, employ satellites orbiting the Earth in a geosynchronous orbit. A satellite uplink communications signal is transmitted to the satellite from one or more ground stations, and then is switched and retransmitted by the satellite to the Earth as a downlink communications signal to cover a desirable reception area. The uplink and downlink signals are transmitted at a particular frequency bandwidth and are coded. Both commercial and military Ka-band communication satellite networks require a high effective isotropic radiated power (EIRP) in the downlink signal, and an acceptable gain versus temperature ratio (G/T) in the uplink signal for the communications link. The EIRP and acceptable G/T require a high gain antenna system providing a smaller beam size, thus reducing the beam coverage and requiring a multi-beam antenna system. The satellite is therefore equipped with an antenna system that includes a plurality of antenna feed horns arranged in a predetermined configuration that receive the uplink signals and transmit the downlink signals to the Earth over a predetermined field-of-view.
The antenna system must provide a beam scan capability up to fifteen beamwidths away from the antenna boresight with a low scan loss and minimal beam distortion in order to compensate for the longer path length losses at the edges of the field-of-view. Multi-beam antenna systems that produce a system of contiguous beams by the plurality of feed horns require highly circular beam symmetry, steep main beam roll-off, suppressed sidelobes and low cross-polarization to achieve low interference between adjacent beams. To provide maximum signal strength intensity independent of the user's orientation, it is necessary that the communications signals be circularly polarized.
To accomplish the above-stated parameters, the antenna feed horns must be capable of producing beam radiation patterns that have substantially equal E-plane and H-plane beamwidths over the operating frequency band of the signal. The level of the cross-polarization and the ratio of the E-plane beamwidth to the H-plane beamwidth in the downlink or uplink signal determines the axial ratio of the signal. If the cross-polarization is substantially negligible and the E-plane and H-plane beamwidths are substantially the same, the axial ratio is about one and the signals are effectively circularly polarized. However, if the E-plane and H-plane beamwidths are significantly different, the signal is elliptically polarized and the received signal strength is reduced, causing increased insertion loss and data rate loss of the uplink or downlink signal.
The useable bandwidth of the downlink signal that is able to transmit information is determined by the combination of the various propagation modes (amplitude and phase) over frequency in the horn aperture. These feed horn propagation modes include the transverse electric (TE.sub.mn) modes and the transverse magnetic (TM.sub.mn).
Traditional, conical shaped feed horns for satellite antenna systems typically limited to a single (TE.sub.11) mode content of the communication signal (uplink and downlink) and had a high axial ratio, and where the E-plane beamwidth was substantially different than the H-plane beamwidth. In order to correct the axial ratio and provide a more circularly polarized beam, Potter feed horns and corrugated feed horns were developed in the art that generated substantially equal E-plane and H-plane patterns with suppressed sidelobes. The Potter horn is disclosed in Potter, P. D., "A New Horn Antenna with Suppressed Sidelobes and Equal Beamwidths," Microwave, J., Vol. XI, June 1963, pp. 71-78. The Potter Horn is a conical-shaped feed horn that includes a single step transition that generates an additional (TM.sub.11) mode for equal E-plane and H-plane beamwidths and suppressed sidelobes. A corrugated horn is a conical shaped feed horn that includes a corrugated structure within the horn from the input port to the aperture that also allows propagation of the TM.sub.11, mode and suppresses the sidelobes.
Although the configuration of the Potter horn is generally successful in providing a desirable mode content with low cross-polarization and suppressed sidelobe levels, the Potter horn generates signals that are limited by their useful bandwidth, on the order of 3%, because of the amplitude and phase relationship of the propagating modes at the horn aperture. The corrugated horn is able to provide wider bandwidth at the higher mode content, but does so at the expense of signal loss. Additionally, the corrugated horn includes significant horn material, and thus is not lightweight and cost effective suitable for the space environment.
What is needed is a compact, lightweight, easy to manufacture, and cost effective antenna feed horn that provides substantially equal E-plane and H-plane beamwidths, low cross-polarization and suppressed sidelobes, but has a higher useful bandwidth than those feed horns known in the art. It is therefore the objective of the present invention to provide such an antenna feed horn.